Heretofore, as this type of a torsional rigidity control device there has been known, for example, an oil pressure variable type torsional rigidity control device for stabilizers disclosed in Japanese Patent Laid-Open No. H09(1997)-156338.
According to this conventional device, stabilizers which connect suspension arms of right and left wheels of each of front and rear wheels are each divided in two at a central part of a torsion bar and one of the thus-bisected portions is fixed to a housing side of a hydraulic rotary actuator (hereinafter referred to simply as “actuator”) for changing a torsional rigidity of each stabilizer, while the other portion is fixed to a rotor side of the actuator.
Corresponding pressure chambers in both actuators disposed on the front and rear wheel sides are respectively brought into communication with differential pressure control valves through conduits, and one conduits communicating with the differential pressure control valves are brought into communication with an oil pressure source through a fail safe valve and a flow dividing valve, while the other conduits are brought into communication with an oil pressure source through a fail safe valve.
Switching solenoids of each differential pressure control valve and fail safe valve are connected to a control device which outputs a vehicle lateral acceleration signal corresponding to the direction and magnitude of a lateral acceleration generated on the vehicle body side.
When a lateral acceleration acts on the vehicle body during travel, the above control device detects the direction and magnitude of the lateral acceleration as a vehicle lateral acceleration signal, then, with this vehicle lateral acceleration signal, switches the fail safe valve from a normal position to an offset position and controls the differential pressure control valves switchingly and correspondingly to the direction and magnitude of the vehicle lateral acceleration signal.
When no lateral acceleration is exerted on the vehicle body such as during vehicular straight running, the control device, while keeping the differential pressure control valves in their neutral position with zero differential pressure under a reference electric current, cuts off the supply of an electric current to the fail safe valve, keeping the fail safe valve in the normal position and allowing the valve to block the actuators provided in the stabilizers for the front and rear wheels, and causes the stabilizers to act as ordinary stabilizers.
On the other hand, when the vehicle begins to turn (cornering) and a lateral acceleration acts on the vehicle body, an electric current is supplied to the fail safe valve in accordance with the vehicle lateral acceleration signal detected by the control device to switch the fail safe valve to the offset position and bring the oil pressure source into an on-load condition, and the differential pressure control valves are put in communication with the actuators.
At the same time, the control device generates a control signal current deviated from a reference value to either positive or negative side correspondingly to the direction and magnitude of the lateral acceleration.
With this control signal current, the differential pressure control valves perform a switching operation by a predetermined amount in a predetermined direction correspondingly to the direction and magnitude of the lateral acceleration exerted on the vehicle body, and differential pressures developed in the differential pressure control valves are controlled and then fed independently to the actuators provided in the front and rear wheel stabilizers.
As a result, the actuators generate moments in a direction corresponding to the direction and magnitude of the vehicle lateral acceleration and, with these moments, a torsional rigidity is imparted to the stabilizers for the front and rear wheels. Then, with a centrifugal force developed at this time, a rolling moment acting in a direction opposite to and competing with a rolling moment acting on the vehicle body is imparted to the vehicle body to effectively suppress the rolling motion generated in the vehicle body.
The torsional rigidity control device described above involves no problems in point of function, but it is desired to remedy the following inconvenience.
In the above torsional rigidity control device for stabilizers, the supply of an oil pressure to pressure chambers is controlled using differential pressure control valves and therefore, there sometimes occurs a case where even if a moment acting in a direction corresponding to the direction and magnitude of a vehicle lateral acceleration, which each actuator must generate, is to be generated, it is impossible to do so.
That is, it is a differential pressure in each pressure chamber that can be controlled, and since the differential pressure is a relative pressure, the oil pressure within each pressure chamber in each actuator connected to the associated stabilizer varies depending on an input provided from a road surface. Therefore, it is necessary to constantly control and maintain a differential pressure in each pressure chamber corresponding to a generated moment. Moreover, because of a differential pressure, an oil pressure required is not determined unambiguously, so that the control becomes complicated and there arises a fear that the oil pressure may become unstable.